This invention relates to a process for producing tubular ceramic structures.
Tubular ceramic structures are known for use as heat exchangers where corrosive liquids or gases are encountered, recuperators, catalyst bodies, as components of fuel cells, particularly solid oxide fuel cells (SOFCs), and in a variety of other applications.
Tubular ceramic structures can be produced in a broad range of lengths, wall thicknesses, and cross-sectional areas and geometries employing any of several known and conventional techniques such as extrusion and dip coating. Each of these techniques for producing tubular ceramic structures generally, and tubular components of SOFCs in particular, is subject to certain inherent drawbacks and/or limitations.
In the case of extrusion, due to the need for the tubular extrudate to remain intact as it emerges from the extruder orifice, the ratio of the diameter of the tube to its wall thickness is typically low, e.g., under 15 and commonly under 10. This practical requirement tends to limit the usefulness of extrusion methods to the production of relatively thick-walled tubular ceramic structures. While relatively thick-walled tubular anodes can be advantageous for the construction of some types of SOFC devices, in particular, those intended for high power output (e.g., 20 KW and above), relatively thin-walled tubular anodes are generally preferred for the construction of SOFC devices of lower power output where their low thermal mass favors quicker start-ups and/or frequent on-off cycling.
The requirement for a relatively thick-walled extrudate, which can only be achieved with an extrudable material of fairly high viscosity, e.g., one of paste- or putty-like consistency, imposes yet another limitation on the usefulness of extrusion methods for the manufacture of tubular ceramic structures, namely, the need to carefully and completely dry the extrudate before subjecting it to such high temperature downstream processes as the burning out of organics (i.e., residual solvent(s), dispersant(s), binder(s), etc.) and sintering. The drying of the extrudate requires suitable control over such operational parameters as temperature, humidity and time. Too rapid drying and/or insufficient drying can result in the production of mechanical defects in the extrudate before and/or after carrying out either or both of the aforementioned high temperature post-extrusion processes.
Still another limitation of the extrusion technique is its inability to readily vary the composition of the extruded tube, e.g., to alter the composition of the tube in one preselected location but not in another.
In the case of dip coating, the requirement that the ceramic-forming composition be applied to a tubular substrate generally limits this technique to the production of structures in which the substrate becomes an integral, functional component of the final article. This requirement for a tubular substrate necessarily restricts the type as well as the design of those devices that can utilize a tubular ceramic article produced by the dip coating technique. Moreover, it is difficult in practice to provide tubular ceramic structures with relatively thin walls and/or with walls of uniform thickness employing dip coating.
There exists a need for a process for producing tubular ceramic structures that is not subject to any of the aforedescribed drawbacks and limitations of known and conventional extrusion and dip coating techniques. More particularly, there is a need for a process which with equal facility is capable of producing tubular ceramic structures over a broad range of wall thicknesses, i.e., from the very thin to the very thick, does not require close attention to and control of the conditions of drying, is readily capable of altering or modifying the composition of the tubular product for a defined portion thereof and does not require the use of a tubular substrate which is destined to become a permanent component of the product.